An exhaust gas from a vehicle, for example, an automobile, contains components such as carbon monoxide (CO), hydrocarbon (HC), nitrogen oxide (NOx), a particulate matter (PM), etc. These components in an exhaust gas may result in environmental pollution, for example, air pollution, smog or acid rain, and the amount of automotive exhaust gas emissions is therefore regulated in various countries.
In order to prevent discharging of these exhaust gas components into the atmosphere, various catalyst devices have been developed. An exhaust gas purification system equipped with such a catalyst device is also known. For example, an exhaust gas purification system is known, in which a Start Converter (SC) catalyst device, a NOx Storage Reduction (NSR) catalyst device, and a Selective Catalytic Reduction (SCR) catalyst device are arranged in this order along an exhaust passage for exhaust gas discharged from an engine.
This system can effectively purify the components above, such as carbon monoxide, even when the exhaust gas is changed among rich, stoichiometric, and lean atmospheres. The rich atmosphere, the stoichiometric atmosphere, and the lean atmosphere refer to atmospheres where the air-fuel ratio is less than the theoretical air-fuel ratio, is the theoretical air-fuel ratio, and is more than the theoretical air-fuel ratio, respectively. For example, a relatively large amount of a reducing agent (e.g., CO and HC) is contained in the exhaust gas under a rich atmosphere; a relatively large amount of an oxidizing agent (e.g., NOx) is contained in the exhaust gas under a lean atmosphere; and the reducing and oxidizing agents which have chemical equivalent amounts are contained in the exhaust gas under a stoichiometric atmosphere.
There is a possibility that the exhaust gas purifying performance of such a system or a start converter catalyst device contained therein can be further improved.
The exhaust gas purification catalyst device of Patent Document 1 includes a first catalyst layer containing an oxygen storage material as a support and Pd; and a second catalyst layer containing an oxygen storage material as a support, Pd and Rh and formed on a surface of the first catalyst layer. In this exhaust gas purification catalyst device, at least a part of Pd particles contained in the first and second catalyst layers is supported on the oxygen storage material.